1. Field of the Invention
This invention relates to a process, an article prepared according to said process and a method employing said article. More specifically, this invention involves a process for preparation of a solid phase dispersion of photoconductive materials within an insulating binder matrix.
2. Description of the Prior Art
The formation and development of images on an imaging member of photoconductive materials by electrostatic means is well-known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging layer of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic powder material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface having a relative polarity opposite to the charge on the toner and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor in the event that the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films where the photoconductive insulating layer is also an integral part of the finished copy. U.S. Pat. Nos. 3,121,006; 3,121,007.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, the developed image is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well-known techniques can be used to permanently affix the toner image to the transfer sheet, including overcoating with transparent films and solvent or thermal fusion of the toner particles to the supportive substrate.
In the most popular of the xerographic systems of the type referred to above, the imaging member comprises a photoconductive insulating layer of amorphous selenium on a suitable conductive substrate. Such photoconductive insulating layers are generally prepared by vacuum deposition of selenium under carefully controlled conditions. These vacuum deposition techniques do not readily lend themselves to the continuous manufacture of photoconductive imaging members. Even under carefully controlled conditions, vacuum deposition of photoconductive insulating layers of amorphous selenium is often beset with difficulties. For example, lack of uniformity in deposition can lead to so-called "pin holes" in the selenium layer. Spattering of molten selenium from the crucible in the deposition chamber has also been known to cause uneven deposition and blemishes in the surface of the imaging layer. Nor is it uncommon for the vacuum deposition chamber to be contaminated with dust particles which codeposit along with the selenium on the receptive substrate thus forming additional imperfections in the surface of imaging layer. In addition to the technique described above, a number of alternative procedures have been disclosed for preparation of selenium and selenium containing films. Representatives of such alternative procedures include the electrochemical deposition of selenium from a suitable electrolyte (U.S. Pat. Nos. 2,649,409 and 2,414,438) and the chemical deposition of a metal selenide film from a solution containing a metal salt, selenourea and other ingredients, Chem. Abstr. 79, 84806j (1973). Although such electrochemical and chemical deposition procedures can provide very precise control over both the rate and uniformity of deposition selenium and metal selenide films, neither system has received general commercial acceptance.
Recently, a number of alternative photoconductive insulating layers have been disclosed wherein a photoconductive pigment is (a) dispersed in a charge carrier transport matrix, (UK Pat. No. 1,343,671, which in turn claims priority to U.S. Pat. application Ser. No. 371,646, filed June 20, 1973) or an electronically inert binder U.S. Pat. No. 3,787,208 or (b) sandwiched between a conductive substrate and a charge carrier transport layer, UK Pat. No. 1,337,228 (which in turn claims priority to U.S. application Ser. No. 94,139, filed Dec. 1, 1970).
In the imaging members disclosed in previously referenced UK Pat. No. 1,343,671, the carrier generation and transport functions are separated and, thus, it is possible to prepare photoconductive insulating layers having less than 10 parts by weight selenium in the imaging layer of the photoreceptor receptor while retaining electrophotographic speed at least comparable to that of amorphous selenium alone. Since the photoactive material, (e.g. a selenium pigment), is dispersed in the insulating resin, the method for preparation of photoconductive insulating layers from such dispersions can follow generally accepted coating techniques applicable to such resinous materials. The simplicity of such procedures can be readily adapted to a continuous manufacturing process thereby increasing the efficiency of preparation of such photoconductors. In preparation of such binder layers, a photoactive pigment and an electronically active insulating binder resins are dispersed in an appropriate solvent and the resulting dispersion cast or coated on a conductive substrate to the desired film thickness. The resulting film contains a random distribution of photoactive particles throughout a charge transport matrix.
Carrier generation and transport functions can also be separated in non-binder photoreceptor systems (UK Pat. No. 1,337,228) simply by overcoating a thin layer of amorphous selenium with an electronically active matrix, such as poly(N-vinylcarbazole). In the dark, the overcoating is sufficiently insulating to support a sensitizing surface charge, (relieving the selenium layer from performing this function), and thus allowing the use of a selenium photogenerator layer of reduced thickness. The overcoating also helps to mask surface imperfections in the selenium layer.
Where it is possible to orient such photoconductive particles within a suitable binder (U.S. Pat. No. 3,787,208) the concentration of such photoconductive pigments can be further reduced without any compromise in the electrophotographic speed of the photoconductive insulating layer. The mechanism involved in the orientation of such photoconductive pigments in the above referenced application is analogous to the situation existing in preparation of ceramic materials from refractory mixtures having a predetermined particle size distribution. In such a system, the smaller particles are forced to occupy the spaces between the larger particles. Although this system provides a degree of control over the spatial distribution of photoconductive pigments within a binder layer, such control is a function of a particle size distribution rather than an ordering of such materials in compliance with a predetermined arrangement or pattern.
The controlled distribution of chalocogenides within an amorphous glassy matrix has just recently been disclosed in German patent application OLS No. 2,233,868 (priority being claimed to U.S. Patent application Ser. No. 163,891, filed July 19, 1971). This German application describes a series of systems wherein a precursor (an "organo-elemento" compound) is initially dispersed in the amorphous glassy matrix and chalcogenides selectively extruded therefrom in response to (a) exposure to imaging energy followed by exposure to development energy; (b) simultaneous exposure to both the imaging and the development energy; or (c) exposure to imaging energy. The glassy matrix within which such chalcogenide deposition takes place must be capable of trapping of the intermediate compounds, radicals and charge carriers generated during exposure to imaging energy in order to enable subsequent thermal development and/or enhancement of the desired chalcogenide deposit. Chalcogenide formation is manifest within the glassy matrix by the appearance therein of a permanent, dense and highly visible deposit.
Accordingly, it is the object of this invention to provide a process for preparation of a solid phase dispersion of selenium in an organic polymeric matrix.
More specifically, it is the object of this invention to provide a process for preparation of a photoconductive insulating layer comprising a solid phase dispersion having randomly dispersed selenium particles throughout an electrically inert polymer matrix.
Another object of this invention is to provide a process for preparation of a photoconductive insulating layer comprising a solid phase dispersion having randomly dispersed selenium particles throughout a polymer matrix which is capable of efficient transport of charge carriers of at least one polarity.
Yet another object of this invention is to provide a process for preparation of a photoconductive insulating layer having substantially two separate and highly localized phases, one phase comprising a photogenerator layer and a second phase comprising a charge carrier transport layer.
Still yet another object of this invention is to provide a series of articles prepared by the above processes.
Additional objects of this invention include the use of these articles in one or more imaging methods.